As robot technology grows, more attention is being paid to educational-support robots that assist in learning. For example, an educational-support robot may support students in their school life or help them learn other languages. However, users tend to lose interest in educational-support robots. To solve this problem, a model of emotional expressions has been proposed in human-agent interaction studies. This model, in which the screen agent is programmed to express emotions autonomously, has proven to be effective for interactions between agents and humans. Thus, this paper proposes an emotional expression model for use with educational-support robots. Using simulations, this paper also investigates the number of emotions that a robot can express.